At least one embodiment of the present invention generally relates to variable speed power tools. More particularly at least one embodiment of the present invention relates to controlling the speed and frequency of electric motors in power tools.
Hand held power tools, such as electric drills screw drivers and the like, use electric motors to power a chuck holding a tool. Such power tools usually include a trigger which is manually operated by a user with the motor being controlled by the user pressing the trigger. Power tools in which the motor and chuck speed are varied based on the amount that the trigger is depressed are known as variable speed power tools. Power tools include motors that are powered by an AC or DC power source that delivers current to the motor. As the user squeezes the trigger, more power is delivered to the motor to cause the shaft to rotate faster. Once the trigger is released, current is no longer delivered to the motor.
Typically, power tools include speed control circuits that use pulse width modulation (PWM) to control the voltage applied to the motor. More specifically, the PWM control circuit rapidly cycles power on and off to the motor. The PWM control circuit controls the duty cycles based on the trigger position. The more the trigger is squeezed the larger the on-time duty cycle is and the faster the shaft rotates.
Power tools often experience high current or stalled conditions when a work load exceeds the capability of the motor or the battery. These conditions create extreme loads on the battery, motor and other electric components of the tool. These conditions also reduce the effectiveness of the tool by damaging the battery, motor and other electric components of the tool.
Conventional power tools exaggerate the negative effects of stalled conditions by including a by-pass contact that, when closed, by-passes the variable speed control. The by-pass contact is closed when the desired power output exceeds a certain point. When the by-pass contact closes, the tool directly connects the motor and battery to deliver all available power to the motor. Under certain conditions the use of a by-pass contact is undesirable because it may damage the battery, motor or other electrical components in the tool. The use of a by-pass contact therefore may lead to a reduced tool life and may also lead to a stalled motor condition.
A need exists for a control circuit that more effectively monitors the electrical condition of the power tool in determining the duty cycle. A need also exists for a control circuit that monitors the electrical conditions of the power tool in determining the frequency of the duty cycle. A need further exists for a power tool controller that provides a maximum amount of power to the motor without damaging the battery and that eliminates or reduces stalled motor conditions.
In accordance with at least one embodiment of the present invention, a control system is provided for driving a power tool, comprising a power source, a motor adapted to drive a shaft, and a power switching unit interconnecting the power source and the motor. The power switching unit applies a pulse width modulated (PWM) drive signal from the power source to the motor. A controller monitors at least one electrical characteristic of at least one of the power source, motor and power switching unit, and adjusts an operating duty cycle of the PWM drive signal based on the electrical characteristic.
One aspect of another embodiment of the present invention is monitoring the voltage of the power source, the motor or the power switching unit. Optionally, the system may monitor the current of the power source, the motor or the power switching unit.
Another aspect of an embodiment of the present invention is the use of a controller that detects a voltage drop across the power source. Optionally, the controller detects a voltage drop across said power source and the motor.
In one embodiment of the present invention, the power switching unit comprises a power MOSFET connected in series between the power source and the motor. The power MOSFET switches between ON and OFF states to vary the pulse width of said PWM drive signal. Optionally, an input lead connected to the controller provides a user trigger signal indicative of a trigger position or a motor speed. Alternatively, the PWM drive signal adjusting the motor speed.
Another aspect of an embodiment of the present invention is the use of a voltage sensor to monitor a voltage drop across at least one of the power source, the motor and the controller. Optionally, the controller determines a target duty cycle representative of a target motor condition selected by a user and sets the operating duty cycle below the target duty cycle or at a value not equal to the target duty cycle. Optionally, the target motor condition may constitute the motor speed or torque. Alternatively, the operating duty cycle may be set from the peak current and time period over which the power source delivers a current at or near the peak current.